1. Field of the Invention
This invention relates to cellular communication systems, such as cellular telephone or personal communication services (PCS), and more particularly relates (a) to a multilayer cellular design in which multiple cellular arrangements (each with an assigned group of frequencies) provides a substantial degree of coverage overlap, and (b) to a method for allocating and transferring calls among the cellular arrangements.
2. Discussion of the Background
Conventional cellular systems have a hierarchical system design. A mobile switching office is attached by voice and data links to a number of base stations, each of which is connected to an antenna with a set of frequencies, each of which can connect to a number of mobile units (HHTs) via a radio channel in its predetermined portion of region coverage. An HHT can be a hand-held telephone or other mobile unit communicating voice or data over an assigned frequency channel to a selected base station. Throughout this specification, when voice communication is discussed, the communication channel created and the communication links could be purely data, voice or hybrid voice and data communication.
The mobile switching office and base stations have the computing power to process communicating an HHT's requests for service and to determine which frequency channel assignment will be initially allocated for communicating with the HHT, as well as any hand-off reassignment of channel and antenna necessitated by the HHT moving beyond the cell of the currently assigned antenna.
A common approach to cellular design is illustrated in FIG. 1 and includes a hexagonal lattice of cells with a single antenna covering each cell. The actual portion of the region covered by an antenna may be slightly larger than the hexagonal cell, as shown by the circular region of radius R in FIG. 1. The overlap of cells at the cell boundaries identifies the cell segments in which conventional systems may hand-off the channel assignment and antenna for an HHT moving across a call boundary. However, this cellular overlap covers only a small portion of the geographical area of a cell.
When an HHT with an assigned channel moves to a new cell where the antenna covering the cell has an available channel, the hand-off changing the antenna and frequency for both transmit and receive is transparent to the user. If the antenna in the new cell has no available channel, the call in progress is cut off, this being an unfortunate problem with current cellular systems.
The frequencies used for channel assignments are limited. In a cellular systems the frequency set allocated to a given cell may be reused at some specified distance such as the distance D shown in FIG. 1. This distance must be large enough so as to not create co-channel interference with HHTs using the same channel in different cells. The distance D in FIG. 1 allows the bold-faced seven-cell cluster to be repeated to cover an arbitrarily large geographical region with all frequencies reused repetitively at the same distance D.
The literature teaches various systems (see, for example, FIG. 4 of "The Cellular Concept," V. M. MacDonald, Bell Systems Technical Journal, Vol. 58, No. 1, Pages 15-41, January, 1979, incorporated by reference herein) of patterns of clusters of distinct frequency sets which may be reused at a certain safe distance from each other.
In cellular systems with such frequency reuse allowing coverage of arbitrarily large regions, there is still a problem in that the number of telephone calls that may be active in a given cell at any moment are limited by the number of frequencies allocated to that cell. Some digital systems have improved the total number of calls possible in each cell by multiplexing calls and employing more complex HHTs. Still, the number of active calls in any given cell is limited. When this number is reached by active calls in a cell and not near the boundary where they could be handed off, any new HHT in that cell requesting service will be blocked. Note that blocking can occur even though neighboring cells have available channels.
One solution to the problem of excessive call blocking and call cut-offs is to reduce the cell size, providing a multitude of low power microcells which increases the total available channels over a geographical region by increasing frequency reusage. However, the power of a microcell cannot be reduced too low or the reliability of communication will suffer. Moreover, decreases in transceiver power cause the background noise to signal strength ratio to grow requiring greater HHT complexity to reject the noise levels incurred. Furthermore, as the call size decreases, moving HHTs will require more hand-offs, increasing system overhead and the chance that moving active HHTS will be cut off. This increases the risk of the entire system or portions thereof going into a "thrashing" situation. During a "thrashing" situation, cut-offs of existing calls become a real risk. The cut-off of an active telephone call is considered more disruptive than the unavailability of a channel to a new request for service. Thus, the mobility of HHTs over the region and the level of background noise serve to yield a practical limit to the minimum cell size that may be provided over a region. When the cell size is as small as practical the inability to operate using a majority of channel capacity without noticeable call blockage is a problem with current systems.
Another solution is to have different sets of frequencies occur with different reuse distances, yielding layers of various size cells, where the smaller size cells possessing increased frequency reuse may serve only a non-contiguous portion of the region supplementing the contiguous cell region of another layer. Such multiple reuse patterns add complexity to the system with the smaller cell portion still susceptible to more background noise and greater need for hand-offs. Furthermore, the number of assignable channels in multiple frequency reuse distance systems may vary so as to provide considerably less capacity in portions of each original cell causing surges in those areas to be more disruptive.
Another problem with current systems is that the boundary area between cells is a portion of the region where relatively small movement of an HHT can necessitate a hand-off, and oscillatory movement of an HHT across a boundary or circular motion around the intersection point where three adjacent hexagons meet can greatly increase the occurrence of hand-off overheads while at best preserving a low grade of signal strength to an HHT at such a local in the region. There is a need in cellular systems to avoid the disruptive behavior of service to HHTs happening at the cell boundaries.
Another problem that exists in conventional systems is that a failure, a repair, or the line of a given antenna, which takes the cell off the air, will result in a dead area of coverage in which no available service can occur in that cell for some period of time, and if an HHT moves into that cell while communication is in progress, the communication will be cut off.
The current cellular system has two service providers, and the direction of PCS service, particularly in metropolitan areas, is to have two or more providers offer competing cellular service over the same broad region. The partition of available channels to a multitude of providers, each operating independently and each subject to the degradations in service previously mentioned occurring at more exaggerated levels, compared to the channels available in each system results in poorer overall service. It is a problem to promote competition in cellular PCS service without degrading the level of service that could be provided by the total channels available.